This invention relates to improvement in the process for the preparation of carbonate esters by oxidative carbonylation of alcohols.
Processes for the carbonylation of alcohols have recognized utility in organic synthesis. For instance, the organic carbonate products of such processes have particular value as reactants in the preparation of isocyanates and polycarbonates, and as alkylating agents.
It is known that oxidative carbonylation of an alcohol (ROH) can be carried out by reaction with carbon monoxide and oxygen, according to the equation: ##STR1## Great Britain Patent Specification No. 1,303,756 discloses such a process for which R is a hydrocarbyl group (particularly alkyl, aryl or cycloalkyl) and the reaction is catalyzed by a complexed compound of a metal in Group IB, IIB, or VIII of the Periodic Table, for example, a complex derived from a copper salt, e.g., Cu.sub.2 Cl.sub.2, and an organic base, e.g., pyridine.
However, Belgian Patent Specification No. 859,272 and also a recent publication (Ind. Eng. Chem. Prod. Res. Dev., 1980, pp. 396-403) point out that such a process has a number of disadvantages. Preference is indicated for use of a simple metal salt, most preferably a copper (I) salt, without a complexing agent. The use of copper (II) salts is said to lead to large quantities of by-products, particularly halogen derivatives when copper (II) halides are used as catalysts.
In either case, these prior art processes are found to be substantially and adversely influenced by the presence of water in the reaction medium. (The detrimental effect of water is addressed in detail on pages 400 and 401 of the publication cited above.) Disadvantages resulting from the presence of water are observed both in the reaction rate and in the selectivity to the desired carbonate. Since water is produced during the carbonylation reaction ( equation I) it has been necessary to terminate reactions according to processes of the prior art at low levels of conversion in order to maintain high rates and selectivities.